We have recently shown that intact epithelial cysts excised from kidneys of patients with autosomal dominant polycystic kidney disease (ADPKD) secrete fluid. This demonstration that fluid secretion is a significant factor in the pathogenesis of a major renal disease emphasizes the need to investigate the biology of fluid secretion by renal epithelial cells. That is the objective of this study. We propose to investigate the solutes, the transport mechanisms and the signal transduction systems that are involved in driving fluid secretion by renal epithelia. We will use a systematic approach, directing our investigation first to a well-characterized cell line, a subculture of Madin-Darby canine kidney (MDCK) cells, and then to primary cultures of normal human kidney cortex (HKC) and to primary cultures of cells obtained from cysts removed from kidneys of patients with ADPKD. These studies will be performed on organized epithelial structures formed by these cells: monolayers grown on permeant supports and cysts formed by cells seeded within a collagen matrix. We will also perform direct studies on excised cystic epithelium. In this study we shall use techniques that tie transport of solute to fluid secretion. We have devised techniques that permit us to measure fluid secretion by monolayers of cultured cells, and to couple measurements of cell volume and the rate of fluid secretion by cysts to measurement of changes in cell membrane potential, intracellular Cl minus concentration or intracellular Ca2+ concentration. We will also use standard Ussing chamber techniques. We argue that these studies which focus on fluid secretion are necessary in order to tie future patch clamp and molecular biology studies of membrane transporters to the mechanisms of fluid secretion and to the pathophysiology of the ADPKD cyst.